Accurate measurement of average flow rate in large line size gas ducts has heretofore been difficult, and often expensive. The available instruments in use today fall into two basic categories: (1) instruments installed directly in the line that measure an average flow rate, such as the orifice, the venturi and flow nozzles; and (2) instruments inserted into an existing line that measure point flow rates and rely on mathematical averaging techniques to obtain the average flow rate, such as hot-film probes, pitot probes, insertion turbine meters, and insertion vortex meters. The instruments installed directly in the line are expensive and cumbersome to install. Insertion type probes are less expensive and easier to install, but have poor accuracy of average flow rate, owing to the use of point measurements to obtain it.